1. Field of the Invention
The present invention relates to a thermal printhead, and more particularly, to a thermal printhead having an optimally shaped resistor layer.
2. Description of the Related Art
Thermal printing techniques have been widely used in such areas as portable/mobile, retail, gaming/lottery, and medical due to several advantages over other types of printing techniques such as inkjet, laser or ribbon. Some examples of the advantages are quiet operation, light weight due to a simple structure, no need for ink, toner, or ribbon to replace, and the like. With these advantages, thermal printers based on the thermal printing techniques are used in a variety of devices under a wide range of environments. In particular, thermal printers are likely to be subjected to a wider range of temperatures compared with other types of printers which are mainly used in offices or in a house. As thermal printers rely on heat to print images onto a thermosensitive paper, there is a need for a thermal printhead used in a thermal printer that can offer a reliable fast printing without deterioration of the printing quality even in an extreme ambient temperature.
FIG. 1 shows a simplified cross-sectional view of a conventional thermal printhead B1. The thermal printhead B1 includes a substrate 101, a resistor layer 102, a heatsink 105, a drive IC 106 and a platen 120. In printing an image using the thermal printhead B1, a portion of the resistor layer 102 which constitutes a heating element to imprint a dot is heated by supplying electrical power. When a series of dots is to be printed, this particular portion of the resistor layer 102 is repeatedly supplied with electrical power with power on times in between power off times and the series of dots is printed onto a thermosensitive paper 121 during the power on times. If the series of dots is a long one, the temperature buildup of the resistive layer 102 may occur. Particularly, when On/Off switching speed of supplying electrical power is increased, it may become difficult for the resistive layer 102 to follow the increased switching speed because the resistor layer 102 cannot dissipate the heat fast enough due to the temperature buildup.
In contrast to forced heating of the particular portion of the resistor layer 102 by electrical power, cooling of the particular portion of the resistor layer 102 occurs by conducting heat through the substrate 101 and by dissipating the heat through the heatsink 105 to surrounding air. In other words, cooling time of the heating element of the resistor layer depends on natural cooling which in turn depends on such factors as the combination of the heat capacity of the resistor layer 102, heat capacity and conductivity of the substrate 101 and the heatsink 105 and an ambient temperature of the surrounding air. If, for example, the heat capacities of the resistor layer 102 and the substrate 101 are too large to dissipate the heat in time to follow the On/Off switching speed, problems such as trailing or a blur of a printing dot may occur. Even if the heat capacities of the resistor layer 102 and the substrate 101 are small, if the heatsink 105 cannot dissipate the heat conducted by the resistor layer 102 and the substrate 101 fast enough, the same problems may occur.
FIG. 2 shows a partial schematic plan view of an example of the resistor layer 102. A resistor portion 103 of the resistor layer 102 or a resistor element constitutes one heating element which is used to imprint a dot onto the thermosensitive paper. The resistor portion 103 has a rectangular shape. In order to imprint a dot, a pulse of electrical energy is supplied from the corresponding individual electrode 104 through the resistor portion 103 to the corresponding common electrode 105 and electrical current flows through the resistor portion 103 in the direction indicated by the arrow X in FIG. 2. This is the direction, a thermosensitive paper moves during printing. The direction indicated by the arrow Y in FIG. 2 shows the direction of a series of the resistor portions 103 of the resistor layer 2 being formed on the substrate 101. These directions indicated by X and Y in FIG. 2, hereinafter will be referred to as the paper moving direction and the resistor layer direction respectively. As the electrical current flows, the resistor portion 103 heats up to imprint a dark image of the dot onto the thermosensitive paper. Because both sides of the resistor portion 103 each facing a gap formed by each neighboring resistor portion can dissipate heat faster than the middle area of the resistor portion 103, the temperature buildup of the middle area occurs faster than the sides of the resistor portion 103.
FIG. 3 shows a typical temperature profile of the resistor portion 103 when the resistor portion 103 is heated. The horizontal axis indicates the temperature and the vertical axis shows position within the resistor portion 103 in the resistor layer direction. As the thermosensitive paper moves above the resistor portion 103 in the paper moving direction, the middle area of the resistor portion 103 where the temperature is highest, will imprint a darker image than other parts of the resistor portion 103.
FIG. 4 shows an example of the imprinted image of a dot by the resistor portion 103 in such a situation. The trailing edge of the imprinted image shows trailing or a blur because the middle area of the resistor portion 103 will take more time to cool down after the supply of the electrical energy is stopped, due to the temperature profile as shown in FIG. 3. This trailing or a blur causes deterioration of the printing quality. Conventionally, one way to prevent the blur was to slow down the printing speed enough allowing the middle area of the resistor portion 103 enough time to cool down in line with the other areas of the resistor portion 103 such that any blur becomes negligible.